A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. Electronic semiconductor devices, magnetic devices, and optical coatings are major applications benefiting from thin-film construction. Atomic layer deposition (ALD) is a thin film deposition technique that is based on the sequential use of a gas phase chemical process. ALD is a self-limiting (the amount of film material deposited in each reaction cycle is constant), sequential surface chemistry that deposits conformal thin-films of materials onto substrates of varying compositions. Due to the characteristics of self-limiting and surface reactions, ALD film growth enables control of thickness of deposited layers at the atomic scale.
Noble metals such as Palladium (Pd) have applications in an array of fields, including hydrogen storage and sensing, chemical catalysis, electrical energy storage, and fuel cells. In each of these areas, formation of surface and subsurface alloys has been identified as a promising avenue for enhancement of properties of the metal. In the case of hydrogen storage and sensing, computational investigations suggest that modulation of surface hydride energetics resulting from the over layer could lead to enhanced kinetics of hydrogen absorption and desorption.
Electrochemical atomic layer deposition involves repetition of a two-step process that, in the first step, exploits the fact that some materials electrodeposit as a monolayer onto a dissimilar substrate at a less negative potential than they would deposit onto themselves. In the second step, a galvanic exchange reaction replaces the sacrificial layer formed in the first step with a material upon which the first step can be repeated. It is typically done in an electrochemical flow cell. When scaled up for substrates with large surface areas, large electrical current sources are required.
In the past decades, several strategies for atomic layer deposition have evolved. However, those strategies for ALD either apply to a limited range of materials, require elevated temperatures or highly reactive precursors that can damage substrates, cannot easily be brought into electrical contact, and/or are not scalable because they require instrumentation that generates an electrical current proportional to the substrate surface area.
Electroless deposition is a method that enables thin film growth under mild conditions on substrates to which electrical current is not easily supplied. This method usually involves a second chemical agent that donates or accepts electrons from the species being deposited. Electroless deposition does not involve a mechanism to constrain growth to one atomic layer at a time. Galvanic exchange, also known as galvanic displacement, galvanic replacement, or redox replacement, is a type of electroless deposition in which the substrate serves as the second chemical agent, typically being oxidized as the desired material is deposited. Except when this is used as a step in electrochemical atomic layer deposition (where a sacrificial atomic layer is first electrodeposited), a galvanic exchange reaction is not limited to an atomic thickness; its extent is limited by a balance between corrosion and passivation of the substrate.